1. Field of the Invention
This invention pertains in general to nuclear fuel element cladding and, more particularly, to a texturized pattern on at least a portion of the nuclear fuel cladding.
2. Related Art
The uranium oxide fuel in a pressurized water reactor (xe2x80x9cPWRxe2x80x9d) is encased in sealed tubes, commonly referred to as the fuel cladding. The cladding maintains the fuel in a position, from which controlled fission can proceed and generate heat. The cladding then transfers the heat from the fuel to pressurized water that circulates around the primary loop of the reactor coolant system. The heated water in the primary loop is used to boil water in a steam generator and the steam is then expanded in a turbine that powers an electrical generator.
The primary loop is pressurized to prevent boiling son the fuel cladding surface. Boiling is undesirable in PWRs because the coolant contains dissolved boric acid and lithium hydroxide. The boron-10 isotope in the boric acid controls the reactor power level by absorbing neutrons. Boiling can alter the boron-10 concentration by selective distribution of the boric acid or its salts between liquid and gas phases. Thus, boiling may cause local changes in the neutron flux and power by producing high local concentrations of boron-10. The lithium hydroxide will also be concentrated at boiling surfaces and depleted from the steam phase. This is undesirable, since lithium hydroxide is used to control pH within a range that is optimal for minimizing corrosion of system materials. Local changes in lithium hydroxide concentration may move the pH of the reactor coolant water outside the optimum pH range, with the result that corrosion is accelerated.
Boiling on a PWR fuel cladding surface is also problematic because it encourages the deposition of corrosion products on the cladding surfaces. PWR coolant always contains small concentrations of iron, nickel, chromium, zirconium and other metals and metal oxides. These are present in the coolant because of corrosion of the steam generator tubing, the fuel cladding, piping and other materials within the reactor coolant system. Deposits on fuel cladding in a PWR are referred to as xe2x80x9ccrudxe2x80x9d. The crud deposited on the fuel surfaces becomes activated and then is transported to other parts of the primary system where unwanted radiation fields develop. Furthermore, the crud further accelerates the boiling process and serves as a site for the collection of boron and lithium compounds.
Despite the possible undesirable consequences of boiling on fuel cladding in PWRs, many U.S. reactors now operate in a mode that produces limited boiling on the cladding in the upper regions of the core. This situation has arisen because the power of fuel assemblies and the temperature of the primary coolant have been increased beyond original design levels to increase electrical output and reduce fuel cost.
The power increases have been small enough to minimize the boiling in the xe2x80x9csub-cooled nucleatexe2x80x9d mode. With sub-cooled nucleate boiling, the cladding temperature exceeds the boiling point that was set by the system pressure (typically, 652.7xc2x0 F. (344.8xc2x0 C.) at 2250 psi (158.2 Kg cm2)). The temperature of the bulk coolant is below the boiling point so steam bubbles that form on the cladding surface collapse as they grow and contact the cooler bulk water.
Typically, the bubbles in low powered PWR sub-cooled nucleate boiling are small and because they form and collapse so rapidly, deposition of boron and lithium usually does not occur at levels that can be detected. However, at some reactors, higher sub-cooled boiling and thick crud deposits have formed that have amplified the boiling concentration beyond that which is found at a clean surface. Boron deposition has reached high enough levels in some plants to cause flux depressions in the top of the core. In other cases, deposits have caused flux depressions and fuel failure. Applicants"" field studies as well as theoretical modeling have shown that deposits over 35 microns thick are required to form boron and lithium rich deposits that flux depressions.
It is, therefore, an object of this invention to provide an improved cladding that prevents the buildup of thick crud on the cladding surfaces, that is crud more than 35 microns thick.
It is a further object of this invention to provide such a cladding that will deposit the crud more evenly over a much larger surface area of the cladding than has heretofore been experienced.
This invention achieves the foregoing objectives by providing a nuclear fuel assembly having an array of a plurality of axially-extending elongated tubular nuclear fuel elements respectively encapsulating a fissionable material within at least a portion of the interior volume thereof. The surface texture of the cladding varies axially in a prescribed pattern along the cladding. Preferably, the portions of the fuel element cladding that experience sub-cooled nucleate boiling during reactor full power operation that is not insubstantial is polished and the portions of the cladding surface that experience insubstantial or no sub-cooled nucleate boiling during full power reactor operation are abraded, to more evenly distribute the crud over the cladding without incurring deposits in excess of 35 microns.
In one preferred embodiment, the prescribed pattern on the outside of the fuel element cladding extends over the upper third of the axial length of the portion of the cladding encapsulating the fissionable material. Preferably, the polished surface of the prescribed pattern does not have defects in excess of approximately 0.1 microns.
In another preferred embodiment, the polished and abraded surfaces of the cladding alternate in the upper third portion.
In still another embodiment, the abraded axial lengths of the cladding are located substantially at or just above at least some of the grid elevations where spacer grids align the fuel elements in the spaced array.
In an additional embodiment, the polished surfaces include at least one hillock or bump to disrupt the laminar flow of coolant.